In the production of petrochemicals, water is often used to control various chemical reactions, for example by transferring heat from process streams to quench reactions. When such water is subjected to intimate contact with a process stream, it is commonly referred to as process water. In ethylene manufacturing plants, steam is contacted with the process feedstock to control the pyrolysis (cracking) process by lowering the partial pressure of the hydrocarbon feedstock, improving the efficiency of the conversion reaction. Downstream of the pyrolysis reactors, a water quench tower is further employed to cool the gas leaving a primary fractionator or a transfer line exchanger.
In pyrolysis processes including ethylene production processes, the “Dilution Steam System” (DSS) typically consists of a series of fluidly connected devices including a quench water tower, an oil/water separator, a process water stripper, and a dilution steam generator. Together, the DSS devices in fluid connection represent a water recycling loop. Steam from the dilution steam generator is sent to the pyrolysis furnace and is recovered as water in the quench tower. Temperatures at the base of the quench water tower can approach 100° C., for example about 60° C. to 100° C., or about 80° C. to 90° C.
The water recycling loop can experience a variety of problems due to conditions and compounds present in the quench water tower. Significant quantities of pygas, pytar and various other contaminants formed as a result of the pyrolysis process can concentrate in the process water of the quench water tower. If unchecked, these contaminants can lead to fouling of equipment, that is, deposition on interior surfaces of the dilution steam system. Lack of effective gasoline/water separation in a quench water tower (QWT) or quench water settler (QWS) leads to pygas, pytar, and reacted or reactive byproducts of these circulating through the DSS. Fouling potential is due to the presence of reactive species such as polymerizable compounds present in the pygas or pygas/pytar, heat removal by “pumparounds”, and a mixture of hydrogen, steam, and generally harsh conditions present during operation of the water recycling loop. For example, in some cases these contaminants or reacted byproducts thereof accumulate on heat exchange surfaces or even proceed into the boiler, where it must be separated from the recyclable water and disposed of as “blowdown”. In some embodiments, as much as 5%-10% of total water volume in the system is blown down. Since blowdown water is also employed to preheat incoming water within the recycling loop, the blowdown water is cooled and this in turn can result in additional precipitation and accumulation of solids on device surfaces.
Byproducts associated with fouling include oligomers and polymers including the residues of one or more styrenes, indene, isoprene, and the like as well as co-oligomers and co-polymers incorporating residues of a variety of other polymerizable compounds present in pygas. Byproducts associated with fouling further include polynuclear aromatic compounds (also known as polycyclic aromatics) such as tars, coke, and coke-like materials arising from chemical reactions of other species present within the dilution steam system.
Due to the presence of pygas within the quench water tower (QWT) or quench water settler (QWS) of such pyrolysis plants, coupled with harsh conditions including high temperatures, byproducts form and can subsequently be carried along with the process water to the process water stripper (PWS). By way of example, capturing styrene in the quench water tower exacerbates fouling by providing conditions favorable for its polymerization, leading eventually to deposition of the styrenic products on equipment surfaces. This causes fouling not only at the bottom of the PWS, but also at the dilution steam generator (DSG) preheaters. This in turn leads to poor energy efficiency and in worse cases, to plant shutdowns due to cumulative fouling. The DSG also fouls due to byproduct carry-over, and the QWT may suffer fouling as well. For plants employing feed saturators in lieu of dilution steam generators, analogous fouling and deleterious results also occur.
Thus, there is a need in the industry to reduce fouling within the dilution steam systems of ethylene-producing or other pyrolysis plants. Less fouling improves the energy efficiency of the system, prevents plant throughput reduction, and prevents product quality issues in the process water due to the use of recycled process water as quench water.